The past decade has witnessed the discovery of forty or more peptides localized in neurons of mammalian brain. Many cases of peptides coexisting in the same neuron with classical transmitters have been described. Our laboratory is investigating the functional significance of coexisting peptides and transmitters in the central nervous system, using behavioral tools. A) We previously showed that cholecystokinin (CCK) potentiates dopamine-induced hyperlocomotion in the nucleus accumbens, where CCK and dopamine coexist. This year, we began to investigate the functional significance of endogenous CCK. We developed a method to electrically stimulate the ventral tegmentum to increase locomotor behavior in a manner analogous to microinjection of DA into the nucleus accumbens. Preliminary data suggest that the CCK antagonist, proglumide, blocks hyperlocomotion induced by VTA stimulation, providing evidence for a functional role of endogenous CCK in the mesolimbic pathway. B) Substance P (SP), corticotropin releasing factor (CRF) and acetylcholinesterase (Ach E) were found to coexist in dorsolateral tegmental neurons projecting to the rat prefrontal cortex. Substance P potentiated, while CRF inhibited, carbachol-induced "boxing". This year we found that substance P antagonists block carbachol-induced "boxing," providing evidence for a functional role of endogenous substance P in the medial prefrontal cortex. C) Oxytocin and cholecystokinin coexist in the supraoptic and paraventricular nuclei of the hypothalamus. Cannulation of postsynaptic sites of this coexistence and evaluation of grooming behavior revealed a competitive interaction between microinjected CCK and OXY. This third case of coexistence, i.e., two peptides with no primary transmitter, appears to have a different mechanism of interaction from the cases outlined in A and B.